1. Field of the Invention
The present invention relates generally to the field of measuring the size distribution of particles and more specifically to a method and system for measuring specific parameters of small particle characteristics.
2. Discussion of the Related Art
A number of methods exist for determining the size distribution of particulate material for particles in the approximate size range of 0.1 to 100.0 microns in diameter. One such method known and used effectively for determining the size of small particles is by sensing and measuring their Brownian motion. Brownian motion is caused by random collisions between the particles and thermally excited molecules of the dispersing media. The velocity and direction of the motion is random, however, the velocity distribution of many particles averaged over a period of time will approach a known functional form. Since small particles are known to move faster than larger particles, the particle size can be determined by measuring the size-dependent velocity distribution. For example, fiber optic Doppler anemometers such as those disclosed in U.S. Pat. No. 4,637,716 to Auweter et al, patented Jan. 20, 1987, and U.S. Pat. No. 4,818, 071 to Dyott, patented Apr. 4, 1989, are capable of measuring the size of very small particles down to a diameter of approximately 0.005 microns. However, such fiber optic Doppler anemometers have been useful for measuring particle size accurately only when all particles are of a uniform size.
One method presently known for measuring the particle size and distribution of very small particles of multiple sizes is disclosed by U.S. Pat. No. 5,094,532 to Trainer et al, patented Mar. 10, 1992. This patent discloses a fiber optic Doppler anemometer and method that directs a beam of light into a scattering medium which contains moving particles. The frequency of the scattered light is compared to non-scattered light emitted from the scattering medium and results in the generation of a first signal having a magnitude which is indicative of the difference in frequency between the scattered light and the non-scattered light. A second signal is generated having a magnitude which varies with frequency on a linear scale. The frequency scale of the second signal is then translated into a logarithmic scale and deconvolved to determine the size and distribution of moving particles within the scattering medium. The translation and deconvolving requires translation of analog signals to digital signals and subsequent processing by a central processor and a vector signal processor using fast fourier transfer techniques (FFT). In order to solve for an entire known particle size distribution of over 80 particle diameters the method just described must sample over 80 frequencies. Even though this method provides an accurate measurement of particle size and distribution, it does require a long time period to process all of the sample frequencies and, therefore, is best suited for use in a laboratory with samples that have been extracted from a process and prepared for analysis. Additionally, the central computer and vector processor required in his method add to its complexity and expense.
The measurement of particle size distribution finds use in the process industries in the manufacture of pharmaceuticals, chemicals, abrasives, ceramics, pigments and the like where the particle size affects the quality of the manufactured product. There is an advantage in the ability to measure particle size in-situ and on-line during the manufacturing process in order to more effectively and quickly respond to any changes in the process that may affect the quality of the finished product and to apply these measurements to a process control system that controls the manufacturing process.